1. Field of the Invention
The present invention relates to a Liquid Crystal Display (LCD) with a touch screen function, and more particularly, to technology for embedding a touch screen into an LCD without a separate touch screen device.
2. Discussion of Related Art
An LCD with a touch screen function receives inputs by finger or stylus on its surface without a separate input unit like a keypad. The LCD with a touch screen is divided into a resistive type, a capacitive type, a Surface Acoustic Wave (SAW) type, and an infrared type according to a data detection method. However, since a touch screen panel is attached to the LCD in each of these types, thickness of a product increases, cost increases, yield decreases, and viewing angle decreases.
To address these problems, technologies for forming a touch screen panel inside an LCD in an in-cell type have been developed, as disclosed in Korean Patent Application Nos. 2006-34878, 2006-15480, etc. In U.S. Pat. Nos. 4,345,248, 5,485,177, 6,995,743, etc., which relate to a method of using leakage current of a phototransistor, technologies for forming both a display region and a photosensitive region on an array substrate are disclosed.
For example, in U.S. Pat. No. 5,485,177, an array substrate includes a display region 100 and a photosensitive region 110 as shown in FIG. 1. Referring to FIG. 1, a photosensitive element 113, a switching element 111, and a charge storage element 112 are formed in the photosensitive region 110. One end of the photosensitive element 113 is connected to the charge storage element 112 and the other end is connected to a scan line 115. When a voltage is applied to the scan line 115 connected to the switching element 111, the charge storage element 112 is charged to a voltage applied from a column line 116 of the switching element 111. Each photosensitive element 113 operates to discharge electric charge stored in the charge storage element 112 depending on illuminance.
However, this operation method has a disadvantage in that the load of the scan lines is heavy since the charge storage element 112 is connected to each scan line 115. When the resolution of the LCD increases, a photosensitive region is not needed for each pixel. In this case, a load difference occurs between scan lines of a pixel with the photosensitive region and a pixel without the photosensitive region, degrading pixel quality of the LCD.
In another example, U.S. Pat. No. 6,995,743 relates to an LCD in which an array substrate includes a display region and a photosensitive region. FIG. 2 is a circuit diagram of a conventional LCD with a touch screen function, and FIG. 3 is a cross-sectional view of the LCD.
Referring to FIG. 2, a photosensitive region 120 of the array substrate includes a photosensitive element 122, a switching element 123, and a charge storage element 121. In general, a phototransistor is used as the photosensitive element 122. Gate and source terminals of the phototransistor are connected to a common line 125. A drain terminal of the phototransistor is connected to the switching element 123 and one end of the charge storage element 121. The other end of the charge storage element 121 is connected to the common line 125. Consequently, the gate and source terminals of the phototransistor 122 are connected electrically. A gate terminal of the switching element 123 is connected to a scan line 126. The other terminals of the switching element 123 are connected to a readout system 127. Referring to FIG. 3, an upper portion of the switching element 123 is covered with a black matrix 128 to block external light, but an upper portion of the phototransistor 122 is open and exposed to external light as indicated by reference numeral 129.
When light is incident on the photosensitive element 120, a current flows through the phototransistor 122 and an output based on a difference of illuminance incident on the phototransistor 122 is read out through the switching element 123.
Here, a maximum readout voltage is proportional to a difference between a gate-source voltage of the phototransistor 122 and a voltage stored in the charge storage element 121 through the switching element 123. The voltage stored in the charge storage element 121 through the switching element 123 is set by the readout system 127 and the gate-source voltage of the phototransistor 122 is a common voltage supplied to the common line 125 of the LCD. The common voltage is a different value according to an LCD operation scheme. For example, in a dot inversion scheme, the common voltage has a value computed by subtracting an offset voltage from a middle data voltage value when an LCD data voltage range is 0˜10 V. When the LCD data voltage range is −5˜5 V, the common voltage has a value computed by subtracting the offset voltage from 0 V corresponding to the middle value. In a line inversion scheme, the common voltage has a value computed by subtracting the offset voltage from 0˜5 V whose polarity has changed when the LCD data voltage range is 0˜5 V.
Consequently, in the conventional LCD with a touch screen function, a difference between voltages applied to both ends of the charge storage element becomes a difference between a common voltage and a voltage applied by the readout system. Therefore, there is a problem in that a maximum readout voltage decreases when the difference between the voltages applied to both the ends of the charge storage element is small.